1) Technical Field
This invention relates to a semiconductor wafer transfer apparatus and, more particularly, to a transfer apparatus for moving semiconductor wafers vertically and horizontally to transfer the wafers from one location to another.
2) Background Art
In the processing of semiconductor wafers to make semiconductor devices, the wafers are subjected to a myriad of processing steps. It is practical to perform many of these steps on a set of wafers positioned in a carrier having a plurality of spaced slots for receiving the wafers in edgewise, coaxial relation with the wafers spaced from each other. Many of these steps may be performed in a carrier made of plastic, which is relatively inexpensive and easy to work with. However, other steps must be performed at high temperatures, such as in a heat treating furnace, at temperatures that cannot be tolerated by the plastic carrier. Consequently, it is common practice to transfer the wafers from a plastic cassette to a carrier made of quartz, often referred to as a boat. Also, it is desirable to transfer the wafers from the quartz boat back to the plastic carrier.
Typically, the silicon wafers in boats are quite close to each other. A typical quartz boat will hold fifty wafers. The quartz boats hold the wafers primarily for heating purposes in ovens, and accordingly there does not need to be much space between the wafers. The spacing in plastic carriers is about half the density, or about twice as great as the spacing of the wafers in the quartz boats. There are normally about twenty-five wafers per plastic boat. The reason for the difference in spacing is that the wafers in the plastic boats will be subject to acid baths or other chemical processing, etc., and thus greater spacing is required between wafers.
The wafers can, of course, be manually transferred, but this is a delicate, tedious process, perhaps requiring the use of a tweezers, with the result that the delicate wafers are often damaged or contaminated through handling. If the damage is immediately detected, this is a serious loss simply because of the cost of the partially processed wafers. If the damage or contamination is not detected until later, in the form of an unacceptable end product or system in which the product is used, the loss is greatly amplified.
In the prior art are also disclosed wafer transfer apparatus which transfer wafers from a first carrier to a second carrier. For example, EP-A-0147435, discloses a wafer transfer apparatus comprising a table with two stations and with a single lifting means which transports the wafers from one station to another and deposits said wafers in the appropriate carrier. This single lifting means is cumbersome and is in use throughout the full transfer process.
The automated system lifts a set of semiconductor wafers from a slotted carrier, transfer them laterally and lowers them into a second slotted carrier. In doing this, jaws on the lifting apparatus open automatically to receive or release the wafers. With a carrier having relatively high slotted sides, such as the commonly used plastic carrier, a pusher engages the lower edge of the wafers exposed through the bottom of the carrier and pushes them upwardly sufficiently far to permit the lifter jaws to receive and lift the wafers. The apparatus is oriented at a slight angle to insure that the wafers are all arranged in precise, spaced, parallel relation so as to cooperate with slots in the lifter jaws and slots in the receiving carrier. The system has the capability to move two sets of 25 wafers to a quartz boat having 50 more-closely spaced slots, and similarly, two sets from a quartz boat may be transferred to two different plastic carriers.
A similar principle is disclosed in U.S. Pat. No. 4,573,851.
EP-A-0163413 discloses an apparatus which transfers semiconductor wafers from and to carriers, in which the wafers are processed. The transfer apparatus includes a movable head having rotatable holder elements which hold the wafers. Wafers are moved out of and into the carriers by vertically moving lifting elements. The lifting elements move the wafers from a carrier to the rotatable holder elements, and vertical lifting elements move the wafer from the rotatable holder elements into carriers after the head has moved itself from one station to another. Typically, there are three positions and stations for the head, and vertical lifting elements are positioned at each station for moving the wafers vertically to and from the carriers which are located at each station.
All these known transfer apparatus do not allow to change the sequence of loading and unloading which is necessary for many purposes in an automated wafer production. Accordingly, there is a need to remedy these drawbacks and to provide an improved transfer apparatus.